Hydrogen production from water splitting is a promising source of clean energy that has the potential to mitigate the crises of fossil fuel dependence and global warming. Today, the industry technology for the hydrogen production is predominantly the steam reforming from hydrocarbons, such as methane, at high temperature up to 1000° C., followed by a water-gas shift reaction. However, this industry process generates one carbon dioxide molecule for every four hydrogen molecules produced. Therefore, it is necessary to develop an alternative green and sustainable energy technology.
Electrolytic water splitting represents a prospective approach to produce hydrogen as carbon-neutral fuel. Compared to steam reforming, this approach has many advantages, such as low temperature, high purity hydrogen, simple and scalable process, and more importantly there is no carbon dioxide emission and electrolytic water splitting uses sustainable source which is water. In general, electrolytic water splitting at acidic condition involves water oxidation on the anode to produce oxygen (or Oxygen Evolution Reaction and denoted as OER) as shown in the half-cell reaction (i) below:2H2O(l)→O2(g)+4H+(aq)+4e−E°=−1.229 V (vs. SHE)  (i)and proton reduction on the cathode to produce hydrogen (or Hydrogen Evolution Reaction and denoted as HER) according to the equation (ii) below:4H+(aq)+4e−→2H2(g)E°=0.000V(SHE)  (ii)
Reaction (i) is the rate limiting step in overall water splitting process because it has higher energy barrier. On the other hand, the proton reduction as shown in reaction (ii) is more favourable as it has much lower energy barrier. When the electrolysis of water is carried out under basic condition, the following half-cell reactions (iii) to (iv) will occur:Anode (OER): 4OH−(aq)→O2(g)+2H2O(l)+4e−E°=−0.401 V (vs. SHE)  (iii)Cathode (HER): 4H2O(l)+4e−→2H2(g)+4OH−(aq)E°=−0.828 V (vs. SHE)  (iv)
Without an efficient catalyst, a significant overpotential is required for water oxidation or reduction, resulting in a low efficiency process. Therefore, the fabrication of an electrode bearing a highly efficient catalyst for water oxidation is the key technology in the development of an economic water splitting process.
Extensive efforts have been made to develop an efficient electrocatalysts for water oxidation application. Recent developments include cobalt and manganese oxide clusters, ruthenium oxide, iridium oxide and nickel oxide based electrocatalysts. Among the oxides of the transition metals, cobalt oxides electrocatalysts have received increasing attention recently due to their earth-abundant nature. However, the electrocatalytic efficiency of these reported catalysts has not reached the level of economic viability. Exploring highly efficient and cost-effective electrocatalysts are still drawing massive investigations around the globe.
As an electrocatalyst for water oxidation, cobalt as a transition metal has a wide range of stable stoichiometries when present as cobalt phosphide (such as Co3P, Co2P, CoP, Co3P2) and thus a high selection portfolio of different chemical phases after reactions. While there are many methods currently available to produce cobalt phosphides, these methods suffer from generation of competing phases and/or lack of size and shape control. In most cases, a mixture of a few stoichiometries is present with a major product and a few by-products. Hence, an additional separation step is necessary to produce the desired cobalt phosphide of high purity with desired crystallinity. Due to many possible variation of stoichiometry, synthesizing cobalt phosphide with a desired composition and high purity is still a huge challenge.
The preparation routes for cobalt phosphides have evolved in the last decade due to the advances in both the synthetic methodologies and the characterization techniques. The most conventional method of preparing cobalt phosphides involves the direct reaction of cobalt metal and highly toxic phosphines or phosphorous pentachloride or triphenylphosphine (PPh3). Subsequently, the previous method has been replaced by reacting cobalt ion with phosphorous, tris(trimethylsilyl)phosphine (P(SiMe3)3). A method using a less reactive and inexpensive phosphorous with controlled shape, size and phase (purity) in the preparation of cobalt phosphides is thus highly desirable.
Therefore, there is a need to provide a method of preparing transition metal phosphide with controlled shape, size and phase that overcomes, or at least ameliorates, one or more of the disadvantages described above.
As the activity of the electrocatalyst is a surface phenomenon, it is critically important to design the proper shape, phase and size of the material. Hence, there is also a need to provide a method of preparing transition metal phosphide-based electrode used for electrocatalytic splitting of water for producing hydrogen that overcomes, or at least ameliorates, one or more of the disadvantages described above.